Helicopters are versatile aircraft in that they are capable of vertical lift and forward propulsion without the need of a runway, unlike most other passenger and cargo transporting style aircraft, such as propeller style airplanes and jet aircraft with a main fixed aircraft lifting wing. Unfortunately, helicopters do have disadvantages that are not generally associated with the other passenger and cargo transporting style aircraft.
One main disadvantage of a helicopter is a limited forward speed. The forward speed is limited due to a limit in available power to satisfy the significantly increasing power demands or requirements needed as forward airspeed increases. One factor that contributes to the large increase in power requirements is referred to as a “retreating blade stall.” During forward flight of the helicopter a section of a helicopter rotor, that is rotating in a rearward direction opposite a flight direction of the helicopter, experiences reversed flow with airspeed that is faster than rotational speed of the rotor. Since the airspeed is faster than the rotational speed of the rotor the rotor begins to “stall”, in the stated section, and results in a large increase in rotor drag and therefore increased power is required.
Another main contributor to the large increase in power required with increasing airspeed is referred to as an “advancing tip Mach number problem”. This is a result of a rotational velocity of a rotor tip, in a direction the helicopter is traveling, experiencing a combination of its rotational velocity in addition to the forward velocity of the helicopter. When the combination of the rotational velocity and the forward velocity exceed a drag divergence Mach number of a corresponding airfoil of the rotor, a large increase in drag is experienced.
The retreating blade stall and the advancing tip Mach number factors are additive and impact power required by the rotor in approximately the same helicopter forward speed regime. These two factors in combination with other lesser contributing factors known in the art, result in limiting forward speed of a helicopter to a speed which is less than a forward speed that is attainable by conventional fixed wing aircraft.
In order to have vertical takeoff and landing capability of a helicopter and to have forward flight speed ability of other aircraft, different styles of vertical takeoff and landing (VTOL) aircraft are being introduced and are becoming more abundant. Generally, dual flight mode VTOL aircraft takeoff as a helicopter with one or more rotating rotors provide lift in a vertical direction.
One style of VTOL that is utilized to overcome the forward speed limitation of traditional VTOL aircraft and to provide increased performance including increased cargo carrying capabilities and increased center of gravity travel capability is a dual rotor (“tandem rotor”) canard rotor/wing design. This canard rotor/wing design includes a pair of wings that operate in a helicopter mode and in a fixed wing mode. Each wing includes two blades that are typically symmetrical and that operate irrespective of flow direction. The canard rotor/wing design provides increased forward speed compared to other types of rotorcraft. The canard rotor/wing design also can provide greater center of gravity travel capability, than single-rotored versions of canard rotor wing aircraft, which allows for greater cargo carrying capability.
The tandem canard rotor/wings may laterally overlap each other, due to their required length and mounting locations. In order to prevent a collision between the rotor/wings the rotor/wings are vertically mounted in an offset configuration such that a first rotor/wing is vertically positioned below a second rotor/wing. However, due to the flex in the rotor/wings a collision situation can still arise between the rotor/wings. To further prevent a collision, mechanical devices may be utilized as they are on tandem-rotor helicopters to maintain synchronization between the rotor/wings such that the blades from the front rotor never lie directly below those from the rear rotor; a nominal 90 degrees of separation is desired, to minimize any possibility of blade-to-blade collisions. These mechanical devices, such as shafts and linkages as used on tandem-rotor helicopters, can be heavy, long, and consume a significant amount of space on an aircraft. The stated mechanical devices are also often exposed to combat damage that can cause loss of rotor synchronization and consequent loss of the aircraft due to blade collisions. As known in the art, it is also generally desirable to minimize the weight of aircraft systems and components to allow for increased carrying capability.
It is therefore, desirable to provide a VTOL aircraft that prevents a collision between rotors without using mechanical positioning systems and that has increased performance including increased cargo carrying capabilities.